Supporting mobility for active users is an essential component of any cellular communication system. In this context, mobility refers to the ability of the system to migrate a user's active connection from one cell to a different cell without interrupting the service, i.e. handover, also referred to as cell change. Handover may take place between geographically non-coinciding cells on the same carrier (intra-frequency handover), between cells on different carriers which may be covering the same geographic area or different areas (inter-frequency handover) or even between different Radio Access Technologies (RATs) which may or may not be covering the same geographic area (inter-RAT handover).
While in active mode, a wireless device, e.g, user equipment (UE), may be commanded to make measurements on the same carrier, different carriers or different RATs, and to report these measurements to a network node when the measurement values exceed a pre-defined threshold. The network node may take into account these measurements and instruct the wireless device to hand over the connection of the wireless device from one cell to another cell. Once the wireless device receives the command to change cells, the wireless device must stop receiving data from the old cell, re-configure with the parameters of the new cell, or re-tune the wireless device receiver in the cases of inter-frequency or inter-RAT handover, synchronize to the new cell and then begin receiving data.
In order to maintain call continuity, the interruption time between the handover command being received and the handover being completed should be minimized such that the wireless device can start receiving data in the new cell. The 3rd Generation Partnership Project (3GPP) wireless communication standard specifications set requirements for the maximum allowable interruption time during a handover for all types of intra-frequency, inter-frequency and inter-RAT handovers for Global System for Mobile Communications (GSM), Universal Terrestrial Radio Access Network (UTRAN) and Enhanced-UTRAN (E-UTRAN) based networks.
In order to improve user experience, so called multicarrier, or carrier aggregation has been introduced into both Universal Terrestrial Radio Access (UTRA) and Enhanced-UTRA (E-UTRA). In UTRA, multicarrier operation enables a wireless device to be configured and simultaneously served from up to 8 High-Speed Downlink Packet Access (HSDPA) cells. Similarly, for E-UTRA, a user may be configured to receive simultaneously on more than one carrier. The purpose of receiving on multiple carriers is to increase the total bandwidth available for serving the user and thus increase the available transfer rates and user experience.
The current 3GPP specifications provide for handing over a wireless device that is being served from an E-UTRA cell in active mode to UTRA. However, currently, the handover can take place to only one UTRA carrier. E-UTRA can operate with significantly higher bandwidths than UTRA, and thus following handover, the user will potentially at first experience a significantly reduced throughput compared to the E-UTRA throughput. For example, if a wireless device is handed over from a 20 MHz E-UTRA carrier to a 5 MHz UTRA carrier, then only one quarter of the pre-handover bandwidth is available after handover. It is possible following handover for the UTRA network to configure multicarrier operation, but this adds additional delays before the previous quality of user experience can be reached.
The situation could be improved by enabling a handover from E-UTRA directly to multicarrier UTRA operation. However in that case, more time may be required by the wireless device to implement the reconfiguration, re-tuning and synchronization for all of the UTRA cells. This additional time could increase the interruption time and the increased handover interruption time could potentially endanger the integrity of the service.